This invention relates to a color picture tube, more particularly three color phosphor films formed on the inner surface of the panel or face plate of the tube, and method of manufacturing the color picture tube.
Generally, a color picture tube is constituted by a panel, a funnel and a neck and dots or stripes of phosphors for emanating red, green and blue colors are disposed on the inner surface of the panel with a predetermined positional relationship. In a recent color picture tube, for the purpose of providing an excellent contrast under bright room light, color filters are provided for respective layers of the red, green and blue phosphors. Such color filters should have such characteristics that they selectively transmit light rays emanated by respective color phosphors without attenuating these light rays as far as possible and that they selectively absorb external light so as to greatly decrease the quantity of external light reflected by the surface of the fluorescent screen thereby improving contrast.
These color filters are generally prepared by the following three methods. According to a first method, as disclosed in U.S. Pat. No. 3,114,065, for example, layers of red, green and blue filter substances corresponding to red, green and blue phosphor layers respectively are interposed between the phosphor screen surface and the inner surface of the glass panel. In this case, the filter substances used a mixture of polyvinyl alcohol, ammonium dichromate and a filter substance, or a mixture of a powder of frit glass and a filter substance.
According to a second method, as disclosed in U.S. Pat. No. 3,886,394, fine particles of filter substances corresponding to respective colors emanated by respective phosphors are deposited on the surfaces of phosphor particles so as to form red, green, and blue filter layers by using these pigmented phosphors.
According to a third method, as disclosed in U.S. Pat. No. 3,114,065 a suspension comprising a mixture of respective phosphors and filter substances is used to cause red, green and blue phosphor layers formed on the glass panel to contain corresponding filter substances thereby forming red, green and blue filter layers.
Where filter layers are formed with one of these three methods, contradictory characteristics of lowering the brightness caused by the intervention of the filter substances and improvement of the contrast degrade overall quality.
With the first method, since the filter layers are disposed between the inner surface of the panel and the phosphor layers the energy of the election emitted by election guns is not attenuated by the filter substances so that the brightness of the light emanated by the phosphors is high. Moreover, as the external light transmitting through the panel is absorbed by the filter substances, contrast can be improved.
However, as it is necessary to form filter layers at predetermined positions of the phosphors for respective colors before forming the phosphor layers the number of process steps increases.
The second method is used widely, and according to this method, so called pigmented phosphors are used in which fine powders of filter substances of respective colors are coated on the surface of phosphor particles by using such binder as gelatin or the like. For the red phosphor, iron oxide or cadmium-sulpho-selenide which are red pigments are used as the filter substance, and cobalt aluminate, or ultramarine which are blue pigments are used for the blue phosphor. According to this method, however, a pigment of one color is incorporated into the phosphor layers of other colors during the step of manufacturing the phosphor surface. Thus for example, red pigment particles penetrate into the blue phosphor layer, and such red pigment absorbs the light emitted by the blue phosphor to greatly decrease the brightness of the blue color.
Although the third method is used to certain extent, since the filter substances are separated from the phosphors, a large quantity of the filter substances in the form of fine particles is incorporated into the phosphor layers of the other colors, thus greatly decreasing the brightness. To eliminate this problem, it has been proposed to interpose a filter substance between one of the red, green and blue phosphor layers and the face plate by first coating a suspension of the phosphor of that color and the pigment. With this method, however, since the filter substance is applied onto the phosphor layer of one color, it is impossible to sufficiently decrease the quantity of external light reflected by the screen surface thus failing to improve the contrast.
FIG. 1 shows the spectrum reflection ratio characteristic of a red filter substance applied to the red phosphor color with a prior art method in which curve a shows the characteristic of cadmium-sulpho-selenide and curve b that of iron oxide. In FIG. 1, the ordinate represents the reflection ratio by taking that of a magnesium oxide, reflection plate of 100%. In the main wavelength range of 610 though 630 nm of the red phosphor, cadmium-sulpho-selenide and red iron manifest a relatively high reflection ratio thus transmitting light emanated from the red phosphor without any appreciable attenuation.
On the other hand, in the blue to green wavelength range of 400 to 550 nm, among of the light rays coming from outside of the screen, light rays in the blue to green wavelength range are selectively absorbed thereby improving the contrast of the screen of the color picture tube.
In the same manner, FIG. 2 shows the spectrum reflection ratio characteristics of a blue filter substance heretobefore applied to the blue phosphor layer in which curve c shows the characteristic of cobalt aluminate and curve d that of ultramarine. The cobalt aluminate and ultramarine manifest relatively high reflection ratio in the main wavelength range of from 420 to 480 nm of the blue phosphor, whereas they show a low reflection rate in the green to red wavelength range of from 530 to 650 nm.
Where a color picture tube is manufactured by using the above described filter substances it is impossible to prevent admixing of a filter substance of one color with the phosphor layers of the other colors at the time of forming a fluorescent screen which degrades the brightness. For example, when the red filter substance shown in FIG. 1 is incorporated into the blue phosphor layer, the red filter substance would absorb the blue color light emanated from the blue phosphor so that blue color brightness decreases greatly. Also when the blue filter substance shown in FIG. 2 enters into the red phosphor layer, the blue filter substance would absorb the red color light emanated by the red phosphor thereby decreasing the red color brightness.